An example conventionally known as a transmission type screen such as is used in projection TV sets has a structure using a combination of a Fresnel lens sheet for converting diffuse light from a light source such as a CRT into parallel beams to be sent to the viewers, and a lenticular lens sheet which performs the function of distributing light only within the range which the viewers watch, for effective use of a fixed amount of light.
A method proposed for forming and processing these lens sheets is the effective formation of a lens sheet in a short time by the irradiation of an energy ray to form a lens layer which has microstructure and is made of a radiation curable resin composition, on a plastic substrate. Since the radiation curable resin composition used for these lens sheets has the characteristics of a high elastic modulus and a high refractive index, the example of using an epoxy (meth)acrylate resin composite has been reported.
The requirements for a lens layer of microstructure made of such a radiation curable resin composition are excellent shape retention withstanding external forces locally applied during the manufacturing process and the like or a change in environment temperature, a high mechanical strength such that disadvantages such as fracture and chipping are not produced upon application of an impact or the process of cutting the sheet, a satisfactory degree of adhesion to the plastic substrate, a high refractive index, and the like.
An example known a radiation curable resin composition for meeting such requirements is a ultraviolet-curable resin composition for a transmission type screen which includes, in specific proportions as essential ingredients, a bisphenol-A type epoxy (meth)acrylate, a bifunctional reactive monomer, a monofunctional reactive monomer and a photoinitiator. This composition has the advantages of providing a cured material with a satisfactory degree of adhesion to a plastic substrate and a high refractive index (see Patent Document 1, for example).
Another known example is an ionization-radiation curable resin composition for a Fresnel lens including a bisphenol-A type epoxy (meth)acrylate, a monofunctional (meth)acrylate such as phenoxy ethyl acrylate, a bifunctional (meth)acrylate such as bisphenol-A tetrapropoxydiacrylate, and a polymer such as an acrylic resin or a polyurethane resin. This composition has the advantages of providing a cured material with a high refractive index, outstanding abrasion resistance and outstanding adhesion to a substrate (see Patent Document 2, for example).
Patent Document 1: JP-A-H5-287040 (pp. 3-6)
Patent Document 2: JP-A-H 11-240926 (pp. 3-4) (U.S. Pat. No. 6,335,079B1)
However, a cured material made of an ultraviolet-curable resin composition for a transmission type screen, which is described in Patent Document 1, has the disadvantages of a propensity for occurrence of indentation creep even after the crosslinking reaction and insufficient shape retention, because a high proportion of a monofunctional reactive monomer from 40 mass % to 70 mass % causes a loosely crosslinked structure resulting from the reaction between a bisphenol-A type epoxy (meth)acrylate and a bifunctional reactive monomer, resulting in a low crosslink density.
Further, a cured material made of ionization-radiation curable resin composition for a Fresnel lens, which is described in Patent Document 2, can exhibit shape recovery properties and shape retention to a certain degree by means of control of a proportion of the content of a polymer component, e.g. a low glass transition temperature polymer such as a polyester polyurethane resin (glass transition temperature Tg: −20° C.), but shape recovery properties and shape retention are insufficient. For this reason, it is difficult to provide the shape recovery properties and the shape retention while concurrently maintaining the moldability such as the casting efficiency of infusing a radiation curable resin composition between the mother mold and the plastic substrate and the shape reproducibility provided by the spread of the resin into the intricate details of the mother mold.
To solve the problems, it is an object of the present invention to provide a radiation curable resin composition for lens sheets use which is outstanding in adhesion to a plastic substrate, shape recovery properties, mechanical strength and moldability, and further shows a high refractive index, and to provide a lens sheet using the resin composition.